The present invention relates to a drive process for photosensitive devices having a matrix of photosensitive points, of the type which are in particular produced by deposition techniques for semiconductor materials, and its object is to reduce or eliminate the remanence effect occurring in the photosensitive points. The invention relates more particularly (but not exclusively) to the driving of such devices which are used for the detection of radiological images. It also relates to a photosensitive device for implementing this process.
The techniques for thin-film deposition of semiconductor materials such as hydrogenated amorphous silicon (aSiH) on insulating substrates, for example glass, make it possible to obtain matrices of photosensitive points which can produce an image from visible or near-visible radiation. In order to use these matrices for the detection of radiological images, it is sufficient to interpose a scintillator screen, between the X-radiation and the matrix, to convert the X-radiation to light radiation in the band of wavelengths to which the photosensitive points are sensitive.
The photosensitive points which form these matrices generally comprise a photosensitive element combined with an element fulfilling a switch function.
The photosensitive element commonly consists of a diode, connected in series with the switch element. The switch element may for example be a so-called switching diode whose xe2x80x9cclosedxe2x80x9d or xe2x80x9conxe2x80x9d state corresponds to the biasing which sets it in forward conduction, and whose xe2x80x9copenxe2x80x9d or xe2x80x9coffxe2x80x9d state corresponds to its reverse biasing. The two diodes are connected with opposite conduction directions, with a so-called xe2x80x9chead-to-tailxe2x80x9d configuration. Such an arrangement is well-known, in particular from French Patent Application 86 14058 (Publication No. 2 605 166) which describes a matrix of photosensitive points of the type with two diodes in xe2x80x9chead-to-tailxe2x80x9d configuration, a process for reading the photosensitive points, and a way of producing such a photosensitive device.
FIG. 1 represents a simplified diagram of a photosensitive device, having a conventionally organized matrix 2. The matrix 2 has photosensitive points P1 to P9, which are each formed by a photosensitive diode Dp and a switching diode Dc that are connected in series with a head-to-tail configuration. The matrix has row conductors Y1 to Y3 intersecting with column conductors X1 to X3 with, at each intersection, a photosensitive point connected between a row conductor and a column conductor. The photosensitive points P1 to P9 are thus arranged along rows L1 to L3 and columns CL1 to CL3.
In the example in FIG. 1, only 3 rows and 3 columns are represented, which define nine photosensitive points, but such a matrix may have a much larger capacity, which may be as much as several million points. It is common, for example, to produce such matrices having photosensitive points arranged along 2000 rows and 2000 columns (in an area of the order of 40 cmxc3x9740 cm), or alternatively arranged along a single line and a plurality of columns in order to form a linear detection array, or else arranged along a single row and a single column in order to form a single photosensitive point.
The photosensitive device has a row drive circuit 3 whose outputs SY1, SY2, SY3 are respectively connected to the row conductors Y1, Y2, Y3. The row drive circuit 3 has various elements (not shown) such as, for example, a clock circuit, switching circuits, and a shift register, which allow it to address the row conductors Y1 to Y3 sequentially. The photosensitive device furthermore has a voltage source 4, supplying the row drive circuit 3 with a voltage VP used to define the amplitude of pulses applied to the row conductors.
In each photosensitive point P1 to P9, the two diodes Dp, Dc are connected together either via their anode, or via their cathode as in the example represented. The anode of the photodiode Dp is connected to a column conductor X1 to X3, and the anode of the switching diode Dc is connected to a row conductor Y1 to Y3.
In the image-acquisition or imaging phase, that is to say the phase of illuminating the matrix 2 with a so-called xe2x80x9cworkingxe2x80x9d light signal, the two diodes Dp, Dc of each photosensitive point P1 to P9 are reverse-biased, and in this state they each constitute a capacitor. It should be noted that the two diodes Dp, Dc are generally designed so that the capacitance exhibited by the photodiode Dp is the larger (by of the order of, for example, 50 times).
During the exposure to a working light signal, the charges are generated in the photodiode Dp by the illumination of the photosensitive point P1 to P9 to which it belongs. These charges, the quantity of which depends on the illumination intensity, accumulate at a point xe2x80x9cAxe2x80x9d on the (floating) node formed at the junction point of the two diodes Dp, Dc. The photosensitive points P1 to P9 are read row by row, simultaneously for all the photosensitive points connected to a given row conductor Y1 to Y3. To that end, the row drive circuit 3 applies a so-called reading pulse of a given amplitude to each row conductor Y1 to Y3 addressed; the row conductors which are not being addressed are kept at a reference potential Vr or standby potential, which is for example earth, and which may be the same potential as that which is applied to the column conductors X1 to X3.
The possible accumulation of charges at the point xe2x80x9cAxe2x80x9d of a photosensitive point P1 to P9 leads to a reduction in the voltage at this point, that is to say a reduction in the reverse-bias voltage of the photodiode Dp. With certain operating modes, the application of the reading pulse to a row conductor Y1 to Y3 has the effect of restoring, to the potential of the point xe2x80x9cAxe2x80x9d of all the photosensitive points connected to this row conductor, the biasing level which it had before exposure to the working light signal: this results in a current proportional to the charges accumulated at the corresponding point xe2x80x9cAxe2x80x9d flowing in each of the column conductors X1 to X3.
The column conductors X1 to X3 are connected to a reading circuit CL, in the example comprising an integrator circuit 5, and a multiplexer circuit 6 which is formed, for example, by a shift register with parallel inputs and series output which may be of the CCD type (charge coupled device). Each column conductor is connected to a negative input xe2x80x9cxe2x88x92xe2x80x9d of an amplifier G1 to G3 connected as an integrator. An integration capacitor C1 to C3 is connected between the negative input xe2x80x9cxe2x88x92xe2x80x9d and an output S1 to S3 of each amplifier. The second input xe2x80x9c+xe2x80x9d of each amplifier G1 to G3 is connected to a potential which, in the example, is the reference potential Vr, which potential is consequently imposed on all the column conductors X1 to X3. Each amplifier has a so-called resetting switch element I1 to I3 (consisting for example of a MOS-type transistor), connected in parallel with each integration capacitor C1 to C3.
The outputs S1 to S3 of the amplifiers are connected to the inputs E1 to E3 of the multiplexer 6. This conventional arrangement makes it possible to deliver xe2x80x9cin seriesxe2x80x9d and row after row (L1 to L3) at the output SM of the multiplexer 6, signals which correspond to the charges accumulated at the points xe2x80x9cAxe2x80x9d of all the photosensitive points P1 to P9.
It should be noted that it is also known, in order to fulfil the switch function which, in the example in FIG. 1, is held by the switching diode Dc, to use a transistor; compared with a diode, the latter involves more complicated connection, but provides advantages in the quality of its xe2x80x9conxe2x80x9d state, which advantages will be explained in the description below.
FIG. 2 schematically illustrates a photosensitive device 1xe2x80x2 which differs from the one in FIG. 1 principally in that it has a matrix 20 in which the switching diodes Dc are replaced by transistors T which are also produced by thin-film deposition techniques (TFTs).
In the diagram shown by way of example in FIG. 2, at each photosensitive point P1 to P9 the transistor T is connected via its source S to the cathode of the photodiode Dp, that is to say to the point xe2x80x9cAxe2x80x9d, its gate G is connected to the row conductor Y1 to Y3 to which the photosensitive point belongs, and its drain D is connected to the column conductor X1 to X3 to which the photosensitive point belongs. The anodes of all the photodiodes Dp are joined and connected to an output SP4 of the row drive circuit 3. The output SP4 delivers a so-called bias voltage VpL which is negative relative to the reference potential VR or earth, for example by of the order of -5 volts, and which is used to form the reverse-bias of the photodiodes Dp; the row drive circuit 3 receives this bias voltage, for example, from a supply source 4xe2x80x2.
In this configuration, the row drive circuit 3 delivers, via its outputs SY1 to SY3, signals or pulses with the same synchronization as in the case of FIG. 1, which signals simultaneously set all the transistors T of a given row L1 to L3 in the xe2x80x9conxe2x80x9d state. At each photosensitive point, putting a transistor T in the xe2x80x9conxe2x80x9d state causes the reference voltage VR to be applied to the cathode of the photodiode Dp: this results, in a manner which is well-known per se, either in initial reverse-biasing of the photodiode (in preparation for an imaging phase)l; or restoring initial reverse-biasing (during a reading phase), with the flow through the column conductors X1 to X3 of a current representing the quantity of charges which are accumulated in the photosensitive points P1 to P9 belonging to the row L1 to L3 addressed. The rest of the operation is similar to that already explained.
The proportionality between the value delivered at the output of the multiplexer 6 and the intensity of the working light signal picked up by a photosensitive point may be distorted for various reasons, among which remanence phenomena are particularly problematic, in particular because they can introduce, when measuring the illumination of a photosensitive point after imaging, a correlation with the illumination of the same photosensitive point during previous imaging.
The greatest cause of remanence in the case of matrices whose photosensitive points are produced from semiconductor materials, and even more particularly in the case of amorphous silicon (aSi), essentially resides in a high density of deep states in the forbidden band of the material: in the case, for example, of amorphous silicon, the lack of a crystal lattice creates traps which can retain charges that are generated during imaging. Under these conditions, the semiconductor material can to some extent xe2x80x9crememberxe2x80x9d an image corresponding to a given illumination process, and return charges relating to an image during the reading of a subsequent image, or even several subsequent images.
With a view to reducing or even eliminating the remanence defect mentioned above, the invention proposes to produce a current making it possible to fill or saturate the traps (or deep states) present in the structure of the semiconductor material, so that these traps become emptied with a statistic which no longer has anything to do with the previous image, which results in a complete absence of correlation and therefore an absence of remanence.
The invention therefore relates to a drive process for a photosensitive device having a matrix of photosensitive points, the photosensitive points being arranged in at least one row and in at least one column and each comprising a switch element in series with a photodiode, the process consisting, on the one hand, in exposing the matrix to a so-called working light signal during an imaging phase, during which charges produced in each photosensitive point according to its exposure modify a bias voltage of the photodiode, and consisting on the other hand in reading the photosensitive points in a reading phase which takes place after the imaging phase, the said process being characterized in that it furthermore consists, at least once before the imaging phase, in firstly exposing the matrix to a so-called erasing light flux having an intensity such that it causes conduction in the forward direction by each photodiode, and in secondly reverse-biasing all the photodiodes.
It should be noted that the process of the invention furthermore makes it possible, straightforwardly, to improve the efficiency with which the photosensitive points are read, particularly when the charges which are accumulated therein have low value. One solution to this problem is known from French Patent Application No. 88 12126 published with the No. 2 636 800. This solution applies to the case in which the photosensitive points each consist of a photodiode connected in series with a diode fulfilling the function of a switch element, and with the two diodes connected in a head-to-tail configuration, as in the example in FIG. 1.
This patent application proposes to create, by additional illumination, so-called drive charges which are added at each photosensitive point to the xe2x80x9csignalxe2x80x9d charges produced by the exposure to the working light signal. This additional illumination can be obtained by various types of light source, for example a lumiplate or an array of light-emitting diodes, as described in a French Patent Application No. 2 598 250.
Referring again to FIG. 1, if the matrix 2 is produced on an insulating substrate 7 (represented by a solid line) which is transparent to light, for example made of glass or quartz as described in the Application No. 2 605 166 cited above, an additional light source SL (symbolized by a dotted line) may be placed against the substrate 7, on the opposite side from the matrix 1 so as not to form a screen against the working light signal. For example, assuming that the substrate 7 lies in a plane which is the same as that of the figure, the light source SL lies in a plane which is deeper than that of the figure. Of course, the device in FIG. 2 may itself also have such an additional light source (not shown in FIG. 2).
The drive charges added to the charges which are created by the working light signal make it possible to minimize the detrimental effect (to very small values) which is produced by the mediocre qualities exhibited by a switching diode used as a switch in the xe2x80x9cclosedxe2x80x9d state, that is to say in the xe2x80x9conxe2x80x9d state. This is due, in particular, to a nonlinearity in the current/voltage characteristic of the diodes, in their mode of conduction in the forward direction. However, this solution which uses an optical flash to produce the drive charges, has the drawback of also producing a high degree of noise (associated with the optical flash).
The process of the invention has the advantage of making it readily possible to add drive charges, with the same aim as that intended in Patent Application No. 2 605 166, but by an electrical method which generates a much lower level of noise than an optical method.
Other characteristics and advantages of the invention will become apparent on reading the following detailed description, given by way of nonlimiting example with reference to the appended drawings, in which:
FIGS. 1 and 2 represent photosensitive devices to which the process of the invention may be applied;
FIGS. 3a to 3e constitute a chronological diagram illustrating the operation of the devices in FIGS. 1 and 2, when being driven using the process of the invention.